Noise suppression circuit



oct. 19, 1937. v 5. M. BROWN' .NOISE SUPPRESSION C IRCUIT Filed Dec. 4, 1935 2 Sheets-Sheet 1 Inventor George M. Brown b 7:/ LJ H/sw/tbornag Oct. 19, 1937. G, M, BROWN 2,096,625

NoIsE SUPPRESSION cmcum Filed Deo. 4, 1935 2 sheetS-sneeiz A/VDE CURRENT 54 A55 Inventor:

' George M. Brow/rw,

bld torrevj.

His t CII Patented Oct. 19, 1937 UNITED VSTATES NOISE SUPPRESSION CIRCUIT' George M. Brown, Scotia, N. Y., assigner to General Electric Company, a corporation of New York Application December 4 17 Claims.

My invention relates to means for suppressing noise in the audio circuit of a high frequency receiver, and more particularly to means for rendering the audio channel of a receiver inoperative in the absence of a received signal carrier. My invention is particularly applicable for use in receivers which operate from an anode voltage source which may vary over rather wide limits, as is the case in police signal communication wherein dynamotors or vibrators operated from an automobile storage battery are employed to supply energy to the plate circuits of the receiver system.

It is an object of my invention to provide an improved noise suppression circuit for a high frequency receiving system which utilizes the change in current produced in the anode circuit of one of the tubes in the system in response to a received signal carrier to control the, bias on the control electrode of one of the audio frequency amplifier tubes in such a manner that when avcarrier is being received the controlled amplifier is biased to its normal operating point, and when no carrier is being received the controlled amplifier is biased to an anode current cut-off value or beyond.

It is an additional object of my invention to provide a circuit of the above character having means connected therein to prevent the grid of the controlled amplifier from being biased further positive than its normal operating point when carrier waves of high intensity are being received, thereby to prevent undesired distortion in the\audio output of the controlled amplifier.

It is a still further object of my invention to provide a circuit having the above characteristics which operates satisfactorily to perform its noise suppressor function irrespective of Voltage variations of the energy supply source for the plate circuits of the system.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and the method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings in which Fig. 1 illustrates a high frequency receiving system having my invention embodied therein; Fig. 2 illustrates the characteristics of the second detector included in the circuit of Fig. 1, Fig. 3 illustrates a modification of my invention; Fig. 4 illustrates my invention as applied to a receiver having a biased detector therein, and Fig. 5 illustrates certain characteristics of the detector included in the circuit .of Fig. 4.

Referring to Fig. l of the drawings, I have illustrated my invention embodied in a radio re- 1935, serial No. 52,859

(c1. 25o-2o) ceiving system having the novely features described in a copending application, .Serial No. 27,896, Stewart Becker, filed June 22, 1935, which application is assigned to the same assignee as the present application. This receiver comprises an antenna 1 from which received oscillations are supplied to the input of a radio frequency amplifier 2 in which they are amplified and supplied to a converter stage 3. The converter stage 3 includes a first detector and local oscillator, which may, if desired, be combined in the same envelope. In order to simplify the drawings, the details of the circuit of the amplifierl 2 and converter stage 3 have been omitted since they are of conventional form and are well' known vin the art. Briefly, the amplified signals from the amplifier 2 are detected and heterodyned with locally generated oscillations in. the converter stage 3. The modulated intermediate frequency output from the converter stage is supplied to an intermediate frequency amplifier 4 through a coupling transformer 5 having its secondary tuned to the operating intermediate frequency by a condenser 6. The input circuit of the amplifier 4 includes the secondary of the transformer 5 connected between the control grid 1 and a cathode 8 of the Aamplifier tube through a by-pass condenser 9, Amplified output currents from the tube 4 are supplied through a coupling transformer I!) to the input circuit of a second intermediate frequency amplifier en,- closed within the envelope of a multi-electrode electron discharge device I I. A tuning condenser I2 is provided for tuning the input circuit of the second intermediate frequency amplifier to the operating intermediate frequency.

The output circuit of the second intermediate frequency amplifier includes a tuned circuit I3 consisting of an inductance I4 and a condenser I5 connected in parallel between the anode I6 and the cathode I'I of the amplifier. A suppressor'grid I8 operating at cathode potential is also included in the amplifier circuit. The inductance I4 is coupled to an inductance I9 connected between the control grid 2U and cathode Il of a second detector having its electrodes enclosed within the envelope of the multi-electrode tube II.

The output circuit of the second detector includes the primary 2I of a transformer 22 having its secondary 23 connected between the control grid 24 and cathode 25 of an audio amplifier enclosed within the envelope of a multi-electrode tube 26. A power amplifier 2l is resistance coupled to the preceding audio amplifier 24, 25 by means of a blocking condenser 28 and a potentiometer resistance 29 connected in series between the cathode 25 and an anode 39 of the tube 26. Amplified audio output currents from the power amplifier 21 are supplied to a loud speaker 3| through aV coupling transformer 32.

A high voltage source is provided for supplying the necessaryenergy to the anodeY circuits ofthe tubes included in the system'. This source is shown as a battery 33 connected across a bleeder resistor 34 having a ground connection 35 at a point intermediateits terminals. The portion of the resistance 34 which is positive with respect to the point 35 is provided with a number of taps 36 and 31 connected to those electrodes of the different tubes requiring less than the maximum positive potential available from the source 33 such, for example, as the screen grids of the two intermediate frequency ampliiiers, the screen .grid of the radio frequency amplifier and the anode ofthe local oscillator included in the converter stage 3.

In accordance with theV invention disclosed in the application referred to above, the advantages of a novel super-regenerative action are attained in the circuit thus far described by regeneratively coupling the output and input circuits of the second detector, supplying a quenching voltage having a low radio frequency value to the input circuit of the second detector, operating the system at a particular intermediate frequency and carefully adjusting the regenerative coupling to obtain optimum performance.

Regenerative feed-back between the circuits of the second detector is obtained by providing an inductance 38 tuned to the operating intermediate frequency by a condenser 39 and coupled to the inductances I4 and I9. For the purpose of obtaining a maximum selectivity of the receiver, the inductances I9 and 38 are loosely cou- Vpled to the inductance I4 and the inductances YI9 and 38 are tightly coupled with respect to each other.

In order to provide the quenching voltage necessary to produce super-regenerative action, an oscillation generator is provided which includes the electrodes 39', 40 and 4I enclosed withinthe envelope of the tube 26. This oscillation generator comprises the tuned circuit 42 connected between the anode 39 and control grid 4U in the conventional manner, the inter'- mediate point of the tuned circuit 42 being connected through a resistance 43 to the positive side of the source of potential 33. Audio frequency currents are by-passed around the source 33 by means of condensers 44 and 45. Oscillations from the anode of the oscillator 39', 40, 4I are supplied to the control grid 29 of the second detector through a coupling condenser 46.y

Although the operating characteristics of the above-described system are such that the signal intensity level is maintained relatively constant in the audio circuit irrespective of the intensity of the incoming carrier, it has been found to be desirable to provide an automatic volume control circuit for maintaining the signal intensity level applied to the second detector below a predetermined value. In the circuit illustrated, the automatic volume control circuit for this purpose includes an anode 41 in the envelope of the tube 4, which anode is connected through an intermediate frequency coupling condenser 48 to the anode of the amplifier in the tube 4 and which cooperates with the cathode 8 of this amplifier to form a diode rectifier operating into a load resistance 48' connected between the anode 41 and theV negative terminal of the supply source 33.

With the above-described` circuit arrangement,

the potential between the point 35 and the'negative side of the source 33 is connected across the cathode Aand anode of the Vautomatic volume control diode thereby'to bias the anode 41 suinciently negative to prevent vcurrent from flowing in the resistance 48 when the received signal has an intensity less than a predetermined value.

- When current flows in the resistance 48 the potential thereacross is supplied through resistance 49 and conductors 50, 5I and 52 to the control grids of amplifier 2, converter stage 3, and ampliner 4 respectively, thereby to reduce the amplication of these tubes upon reception of signals having an intensity greater than the predetermined value.

.The primary function of the conventional automatic volume control circuit described above is to maintain the received signal intensities supplied to the second detector below a value at which overloading of the second detector occurs. Thus, while this conventional automatic volume control circuit may, if desired, be adjusted in the same manner in which it is adjusted for conventional automatic volume control i. e. for minimum practical variation of signal intensity suppliedrto the second detector, when employed in connection with the super-regenerative detector described it is only essential that it reduce the strong signals sufficiently to avoid overloading of the second detector since the principal automatic volume control action is obtained through the inherent operation of the second detector.

In accordance with my invention a network is provided for suppressing noise in the audio channel of the receiver during those periods when no carrier is impressed on the control electrode of the second detector. The noise suppressor network illustrated is particularly applicable to the receiving circuit described above, sinceit utilizes the characteristics of the super-regenerative second detector in performing its noise suppressor function. This network comprises a resistance 53 connected in the anode circuit of the second detector I1, 20 and is arranged to transfer through a resistance 55 the potential of its terminal 54, furthest removed from the bleeder resistor 34, to

the control electrode 24 of the audio amplifier 25,

24, 30. Preferably, a voltage dropping device 56 is inserted between the potential point 54 and the terminal 58 of a resistance 55 for bringing the potential applied to the control grid 24 through the resistance 55 and winding 23 closer to ground potential.

The device 56 may comprise a gaseous discharge device, such, for example, as a neon bulb, having the well-known operating characteristic of a substantially constant voltage drop between its terminals irrespective of small variations in the current iiowing through the tube. If a neon tube be employed as the voltage dropping device, it is desirable to connect a resistor 51 between the junction point 58 and the negative terminal of the source 33 thereby to maintain the neon tube 56 in a conducting state at all times. In order to raise the potential of the cathode 25 to a value such that the potential difference between the cathode 25 and the control grid 24 will be within the normal operating range of the amplifier, a pair of resistors 59 and 60 are provided, which are connected in series between the cathode 25 and ground. As thus connected the current flowing in the anode circuit of the amplifier 30, 29, 25 plus the current flowing in the anode circuit of the oscillator 39', 40, 25 produces a drop across the resistors 59 and 60 which raises the potential of the cathode to a positive value, above ground potential, determined by the resistance values of the two elements 59 and 50.

The operation of the noise suppressor network Adescribed in the preceding paragraphs will best be in the anode circuit thereof decreases. This pro? duces an increase in the potential at the point 54 because of the reduced voltage drop across the resistor 53 connected in the detector output circuit. It has been found that this potential swing is suicient toswing the potential of the grid 24 from beyond cut 01T bias to normal operating bias when a carrier wave is received or is tuned in, even though the carrier may be very weak. Stated in another manner, the positive potential of the point 54 increases when a signal is received and decreases a corresponding amount when the signal is tuned out or completed. This change in positive potential of the point 54 is transferred directly to the junction point 58 by the constant voltage dropping device 56 and is applied directly to the grid 24r of the audio ampliers 30, 24 and 25 through the resistance 55 and winding 23.

If the potential at point 54 be properly adjusted with respect to the potential of the cathode 25, the audio ampliiier 3G, 24, 25 may be biased to cut-01T, or beyond by the potential diierence therebetween when no signal carrier is being received. With the circuit in this condition, if a carrier be received the positive potential of the point 54 is increased in the manner described above. Since this increase is impressed on the grid 24, the negative -bias of the grid 24 with respect to the cathode 25 is decreased by a similar amount which is sufficient, with a usable signal strength, to bring the bias of the grid 24 to a value such that the ampliiier is biased to a normal operating condition. The degree of bias of the grid 24 with respect to the cathode 25 is determined by the voltage drop across the respective resistance 53, 59 and 60 and when proper resistance values are selected for these resistors, the voltage swing produced at the terminal 54 of the resistance 53 in response to variations of the currentin the anode circuit of the detector renders the audio channel operative when a carrier is being received and renders the audio channel inoperative when no carrier is being received. In this manner noise in the output circuit of the receiver is eliminated during those periods when no signal is being received.

It has been found that with the above circuit arrangement when an incoming carrier of high intensity is being received the grid 24 may be biased less negative with respect to the cathode 25 than its normal operating bias due to the low value of current in the anode circuit of the detector, and the correspondingly small voltage drop across the resistance 53 which results in a relatively high positive potential at the point 54. This action is not desirable since distortion of the audio output of the amplifier 35, 24, 25 occurs When the grid 24 attains a less negative bias with respect to the cathode 25 than its normal oper? ating bias.

In order to obviate the above diiiiculty a rectiiier yIlfozf any suitable type is provided' which is connected between the junction point 58 and a junction point 62 between the resistances 59 and E0. This rectier is poled in such a direction that when the point 58tends to become more positive than the point 62, current iiows through the rectifier and when the point 58 is negative with respect to the point 62 current flow is prevented. Thus, the rectifier' 6i functions to prevent the: point 58v and the grid 24 from attaining a positive potential greater than the point 62 irrespective of the intensity of the incoming carrier and, since the cathode 25 is at al1 times more positive than the point 62 because of the voltage drop across resistor 59, a negative bias on the grid 24 with respect to the cathode 25 is insured. It will be observed that in the above described circuit the positive potential of the cathode 25 is dependent upon the magnitude of current flowing through the resistors 59 and 60. This current is comprised of two components, namely; the anode current of the amplier 30, 24, 25 and the anode current of the oscillator 39', 40, 4|. Obviously, as the bias of the amplier 3U, 24, 25 is increased to approach a cut-off value the anode current of the amplier approaches zero and the potential of the cathode 25 is decreased. However, the ampliiier anode current is small as compared to the oscillator current and consequently the change in current produced by the change in bias of the amplifier causes a relatively small change in the potential of the cathode 25. Thus, the oscillator 39, 40, 4| tends to stabilize the potential of cathode 25 and maintain the value thereof Within desired limits. 5

From ythe foregoing description it will be apparent that I have devised a simple noise suppressor circuit which is positive and reliable in its operation to change the audio channel of a receiver from an inoperative to an operative condition in response to a received signal. Although the circuit arrangement described operates in a highly satisfactory manner when the source of anode potential 33 remains constant, it has been found that if the voltage of the source 33 varies, the noise suppressor circuit is affected in an undesired manncr. Thus, if the voltage of the source 33 increases above its normalvalue, the potential at the point 58 corresponding to the potential of the grid 24'is increased in a posif tive direction andthe positive potential of the cathode 25 ,isl increased. Due to the presence of thev device 55 in the circuit, the increase in the positive potential of the grid 24 is greater than the increase in the positive potential of the cathode' 25. This produces a decrease in the biasing potential difference between the control' electrode 24 and the cathode 25, which decrease may be suiiicientto render the amplifier operative to amplify noise when no signal is being received. Iiy onk the other hand. the voltage of the source 33 decreases below normal the negative'bias of the grid 24 with respect to the cathode 25 maybe increased tosuch anl extent that the decrease in biasproduced in `response "tov a received signal may beinsumcient to render the'ampliiier 30, 24, 25 operative.

Referring to Fig. 3 of the drawings I have shown a modification of my improvedv noise suppressor circuit which includes means whereby the noise suppressor function. is rendered unaffected by variations in the voltage of the anode supply source.

'I'his circuit is lshown as including a portion of the'circuit of Fig. 1 and similar circuit elements included therein are identified by the same reference characters. l In the'circuit of Fig. 3the resistance between the junction point 54 and the anode current supply potentiometer Y314 is divided into two sections 63, 64 having-a junction point 65 therebetween. In order to prevent variations in the voltage impressed on the potentiometer resistance ,34 from aecting the action of the noise suppressor circuit an additional circuit is provided, which circuit includes a resistor 66 and a pair of gaseous discharge tubes 61 and 68.

rIhe stabilizer circuit described above functions in the fol1owing.manner:-If the voltage of the source increases, the positive potential of the point 54 tends to increase thereby to increase the positive potential of the point 58 and of the control grid 24. Simultaneously with this action the voltage drop across the resistors 59 and 60 tends to increase thereby to increase the positive potential on the cathode 25. In the absence of the connection 66, 61, 68 the increase in the positive potential of the grid 24 would be greater than that of the cathode 25, and accordingly, the

negative bias ofthe grid 24 with respect to the cathode 25 would be decreased in the manner previously described. Howeverl by providing the series circuit 66, 61 and 68 the circulating current flowing through the resistor 64, resistance 66, and the discharge tubes 61 and 68 and the resistor 68 is increased in response to the increased source voltage and the voltage drop across the resistance 64 is increased, thereby to lower the positive potential at the point 54'and increase the positive potential at the point 62. Due to the. constant voltage drop characteristic of the gaseous discharge tubes 61 and 68, the current owing throughv the resistance T66 increases in greater Yproportion than the increase in voltage across potentiometer 34 and, consequently, the increase in the drop across resistance 60 is disproportionately larger than the increase in voltage of the supply source. As a result the increase in positive potential of the point 62 is greater in proportion than the increase in potential of the voltage source. This amplified increase of potential at point 62 produces a correspondingly'amplified increase in the positive potential of the cathode'25 at the same time that the amplied increase in the positive potential of the control grid 24 occuLrs.

If the voltage of the supply sourcey decreases below its normal value, the 'circuit including elements 66, 61, 68 functions to produce a dispropor-` tionately greater decrease in the positive potential of the cathode 25 which accompanies and compensates for the disproportionately greater decrease in the positive potential ofV the grid 24. Thus, the function of the circuit 66, 61,'68 with a decrease in the voltage of the supply source is the converse of that attending an increase in the voltage of the supply source, i. e. the decrease in positive'potential at point 58 is partially compensated for by the decrease in voltage drop across resistor 64,.and the decrease in positive potential of point 62 is amplied by the action of the gaseous 'discharge tubes 61 and68 to cause a relatively greater decrease in currentV ilowing through the resistance 60.

In the above-described manner the potentials of the grid 24 and cathode 25 are made to vary together in the same direction and by a greater amount than the amount of variation of the voltage of the supply source irrespective of whether:

the supply source voltage increases or decreases from its normal value. Y By providing resistances Stand 66 Vand gaseous discharge tubes 61 and 68 ply source.

having proper resistance values the positivepotentials existing respectively on the grid 24-and such a way that the desired biasing potential exists irrespective of voltage variations of the sup- Further, by properly adjusting resistors Gland 66, the potential of thecathode 25 maybe made to vary by an amount'greater than the potential change of the grid 24, in which case 66, 61, 68 functions to render the operation of the noisevsuppressor circuit independent of supply source voltage fluctuations and in addition functions to compensate for changes in the gain of the receiver produced by such voltage fluctuations.`

Although I have described my invention as being particularly applicable for use in a receiver having a super-regenerative, or grid leak, detector embodied therein, it will readily be understood that my invention may be used with equal success'in a receiver includinga detector of the biased grid type, withV but a slight modification of the circuit arrangement shown. Thus, in Fig. 4 AI have illustrated an arrangement having my improved vnoise suppressor circuit connected therein which includes a biased detector of a'type well known in the art. .As shown, this circuit comprises a triode detector 69 which may have its input circuit coupled to a source (not shown) `of signal modulated carrier current by means of a coupling transformer 1,0. The output circuit of thedetector 69v includes the'primary winding 1I of a coupling transformer 12 having its secondary 13 connected between thecontrol electrode 14 and cathode 15 of an audio amplier 16. 'I'he biasing potential for the control electrode of the detector 69 is supplied by a small C battery 11 shunted by a by-pass condenser 18. A suitable source of anode potential 19 is provided for supplying energy to the respective anode circuits of the detector 69 and amplier 16, .and is shown as shunting the two resistance potentiometer ele- In the noise suppressor circuit for the biased detector arrangement of Fig, 4, a resistance 8| is connected in the anode circuit of the detector Y 69 having its upperterminal 82 connected to supplya control potential to the control grid 83 Aof a triode 84 through a gaseous discharge tube 85 and a resistance 86. A resistance 81 shunted by a Vby-pass condenser 88 is provided for biasing the cathode 89 of the triode 84 to a relatively high positive potential slightly greater than the positive potential impressed on the control grid 83 by the resistance 8|. The anode circuitlof the triode 84 includes a resistance 90 having its upper terminal 9| connected to a second voltage dropping gaseousdischarge tube 92. existing at the terminal 93` of the tube 92 isim'- pressed on the control electrode 14 of the amplii'lerf16 through a resistance 94. A pair of resistances 95 and 96 respectively, are connected in the circuit to maintain the gaseous discharge bulbs 85 and 9| in a conductive condition at all times. A cathode biasing resistor 91 shunted by a by-pass condenser -98 is provided for raising the positive potential of the cathode 15 toV a value slightly above that of the grid 14.Y In order to maintain the current flowing through the resistors 81 and 91 substantially constant irrespective of the magnitude of anode current in the output circuits of the tubes 84 and 16, thereby to maintain the potentials of the cathodes 89 and 15 substantially constant, a pair of bleeder resistors S9 and |80 are provided which cooperate with the biasing resistors 81 and 81 respectively to form current circulating paths. By selecting elements 99 and |08 having suitable resistance values, the circulating current may be maintained at such magnitude that changes in current produced by a decrease in anode current of tube 16, as the control bias is varied between cut oi and normal values, causes only a slight change in the positive potential of the cathodes. Thus, it will be seen that the bleeder resistors 99 and |00 perform the same function in the circuit of Fig. 4 as the oscillator 39', 40, 4| of Figs. 1 and 3.

The operation of the above-described circuit arrangement will best be understood by reference to Fig. 5 wherein there is shown a curve B indicating the relation between anode current and signal intensity for a biased type of detector. It will be seen that when a signal is impressed on the input circuit of the detector 69 the current in the anode circuit thereof is increased. This produces a corresponding decrease in the positive potential existing at the point 82 of the noise suppressor circuit due to the increased drop across the resistance 8|. This decrease is impressed by the tube directly on the grid 83 of the triode S4Ithereby to increase the bias on the grid 83 and decrease the current owing in the anode circuit of the tube 84. This decrease in anode current produces a corresponding increase in the positive potential existing at the point 9| of the circuit, which increase is transferred directly to the control grid 14 of the amplifier 16 by the voltage dropping discharge tube 92. It will readily be seen that withthe grid 14 of the amplier 16 biased to cut-off or beyond at the time the signal is received, the increase in positive potential produced by reception of the signal Will swing the bias on the grid 14 by an amount such that the amplifier 18 is rendered in a normal operating condition. It will further be seen that if the sign-al ceases or is tuned out, the current iiowing in the anode circuit of the biased detector will decrease to its original value thereby increasing the positive potential at the point 82 and causing the noise suppressor circuit to bias the amplifier 1S to cut-01T or beyond.

Although I have described my invention as including a voltage dropping device for reducing the positive potenti-al from a relatively high value existing at a pointin the anode circuit of the detector to a potential somewhat closer to ground potential, it will readily be understood that if the characteristics of the controlled amplifier permit, the use of such a voltage dropping device may be dispensed with. Thus, considering the arrangement shown in Fig. l, for example, if the tube 26 possesses a characteristic such that the voltage difference between the cathode 25 and the cathode heater may approximate the voltage difference between the anode of the detector 28, |1 and ground, the gaseous discharge tube 56 may be omitted from the circuit. With this modified arrangement the cathode 25 may be raised to approximately the anode potential of the detector 28, |1 by suitable resistors 59 and 68 and the point 54 connected directlyto the point 58. However, itis wellknown that the ordinary commercial electron dischargetube at present employed in receiver circuits will not permit such a high volt` An alternative arrangernentwhich may be em r ployed is that of supplying power to the heater for cathode 25 from a separate source, such asa separate transformer winding or battery, in which case the potential of the heater may be kept approximately the same as that of cathode 25. If

this be done Vthe cathode of the controlled amplier may be operated at a value slightly more positive than the point 54 and the point 58 connected directly to the point 54. Obviously such lan arrangement eliminates the necessityfor employing the voltage dropping device 56 and the resistance 51.

While I have described what I consider to be preferred embodiments of my invention, it will of course be understood tha-,t- I do not wish tol be limited thereto since Ymany modifications may be made both in the circuit arrangement and in instrumentalities emp-loyed, and I contemplate by the appended claims to cover all such modications as fall within the true spirit and scope of my invention. y

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. The combination, in a high frequency receiving system, of an electron discharge device including an anode and a cathode, an anode circuit connected between said 'anode and cathode and having a source of potential connected therein, a resistance connected in said circuit, anamplier connected in said system having a control electrode, a circuit for directly impressing the potential of the terminal of said resistance'near-Y est said anode on said amplier control electrode thereby to control the magnitude of negative bias of said control electrode with respect to said amplier cathode, said negative bias being suicient to render said amplier inoperative in the absence ofa received carrier, means responsivev to a received carrier for Varying said potential thereby to change the negative bias of said control electrode and render said ampli-fier operative,

and means for preventing said negative bias from decreasing below a predetermined minimum value1 2. The combination, in a high frequency receiving system, of an electron discharge device including an anode and a cathode, an anode circuit between said anode and cathode having a source of potential connected therein, anamplier connected in said system and including a .control electrode and a cathode, meansl for biasing said amplifier cathode to a relatively high positivel value, a circuit for deriving from said anode circuit a-positive potential slightly below thatv of said amplier cathode and for 'impressing said potential on said control rgrid thereby to bias saidj control grid negative with respect to said amplifier cathode, said negative bias being' suiiicient to render 4said amplier inoperative in the absence of a received carrier, means responsive to arrelceived carrier for varying said potential thereby to` change the negative bias of saidcontrol electrode and render said amplifier operative, and means for preventing said negative biasfrom decreasing below a predetermined minimum value.`

3. The combination, in a high frequency receiving system, of an electron discharge device including an anode and a cathode, an anode circuit connected to said discharge device having a source ofp'otential connected therein, an'amplicontrol electrode and a cathode, a circuit for biasing said amplier cathode to a relatively high positive value, a circuit for'deriving fromrsaid anode circuit a positive potential slightly below that ,of said amplifier cathode and for impressing said potential on said control grid thereby to bias said control grid negative with respect to said amplier cathode, said negative bias being sufcient to render said amplifier inoperative in the absence of a received carrier, means responsive to av received carrier for varying said derivedV positive potential thereby to change the negative bias of said control electrode and render said amplifier operative, and means including a unilateral conducting device `connected between a point in said last named circuit and a point in said amplifier cathode biasing circuit for preventing said Vnegative bias from' decreasing below a predetermined minimum value.

4. In combination, a high frequency receiving system, vincluding a plurality of ampliers each having an anode circuit, a current source for supplying energy to said anode circuits, said source having a variable voltage, a noise suppressor circuit for biasing one of said ampliers toan inoperative condition in the absence of a received carrier and for biasing said controlled amplifier to an operative condition when a carrier is being received, and means connected in said noise suppressor circuit for varying the bias of said controlled amplier to compensate for changes in the gain of said system produced by variations in the voltage of said supply source.

Y 5. In combination, a high frequency receiving system, including a plurality of amplifiers each having an anode circuit, a current source for supplying energy to said anode circuits, said sourcehaving a variable voltage, a noise suppressor circuit for biasing one of said ampliiiers to an inoperative condition in the absence of a received carrier and for biasing said controlled amplifier to an operative condition when a carrier is being received, ,and means connectedin said noise suppressor circuit forvarying the bias of said controlledk amplifier to compensate for changes in the gain of said system produced by variations in the voltage of `said supply source and for preventing variation in the voltages of said supply source fromaiecting the action of said noise suppressor circuit.

6. The combination, in a high frequency receiving system, of an electron discharge device including an anode and a cathode, an anode circuit connected to said discharge device between said anode and cathode, an amplier connected in said system having a control electrode and an anode, an anode circuit connected to said arnplifier, a current sourcefor supplying energy vto said anode circuits, a. circuit for deriving from said rst-named anode circuit a potential which varies in accordance withthe intensity of an in-V coming lsignal and for impressing said potential on said amplier control electrode thereby to control the magnitude` of said control electrode negative bias with respect to said amplier cathode, and means responsive to variations in the voltage of said supply source for changing said control grid bias thereby to oompensatersaid system for changes in the gain of said system produced by said voltage variations.

7. The combination, in a high frequency receiving system, of anielectrondischarge device including an anode and a cathode, an anode circuit connected to salddischarge device between said anode randjcathode, an amplifier connected in said system having a control electrode `and an anode, an anode circuit connected to said amplifiera current source'forrsupplying energy to said anodecircuits, a circuit for deriving from` said first-named anode circuit a potential which varies in accordance with the intensity of an incoming signal and for impressing said potential on said amplier control electrode thereby to control the magnitude of said control electrode negative bias with respect to said'amplier cath--V ode, means for preventing said negative bias from decreasing below a predetermined value, andV `current source for supplying energy to said anode circuits, a noise suppressor circuit for deriving from said first named anode circuit a potential which varies in accordance with the intensity of an incoming signal and for impressing said potential on said amplifier control electrode thereby to control the magnitude of said control grid negative bias with respect to said cathode, means for preventingV said negative bias from decreasing below a predetermined value, and means for preventing variations inthe voltage of said supply source from aiecting the action of said noise suppressor circuit.

9. The combination, in a high frequency receiving circuit, of means to vary the potential at a certain point in said circuit with respect to ground as the intensity'of received signal varies, an amplifier for received signals having a control electrode, a cathode, and a circuit therebetween, a rst point on Vsaid circuit between said control electrode and cathode being maintained at fixed potential with respect to ground, and means to maintain the potential at a second point on said circuit between said control electrode and cathode at substantially xed potential with respect to said rst mentioned certain point for all intensities of received signals and during absence of received signals, the variations in potential between said rst and second points with variations in received signal being suicient to vary the condition of said amplifier from that of substantial inoperativeness to one of normal operativeness.

f 10. The combination, ina high frequency receiving circuit, of means to vary the potential at' a certain point in said circuit with respect to groundas the intensity of received signal varies, an amplier for received signals having a control electrode, aV cathode, and a circuit therebetween, a rst point on said circuit between said control electrode and cathode being maintained atI-xed potential with respect to ground, and means to maintain the potential at a second point on said circuit between said control electrode and cathodeat substantially fixed potentialV with respectto said certain point for all intensities of received signals and during absence of received signals whereby the'potentialvariations between said certain point and ground appear between said iirst and second points, and means utilizing said potential variations to render said amplifier inoperative during reception of signals of less than predetermined amplitude and operative for ampliiication of said signals when said signals have greater than a predetermined intensity.

1l. The combination, in a radio receiver, of a pair of electron discharge devices, one of said discharge devices passing anode current having intensity variable in accordance with the intensity of received signal and the other of said devices having a control electrode, a source of operating voltage, connections for said anode current and said control electrode to said source of operating voltage, a resistance in the connection for said anode current, a resistance in said control electrode connection, and means to maintain a substantially constant potential between the termini of said two resistances remote from said source of operating potential during the variations in intensity of received signal encountered during practice and during absence of received signals, and means including said last means to control said other discharge device in accordance with the intensity of said anode current.

12. The combination, in a radio receiver, of a pair oi electron discharge devices, one of said discharge devices passing anode current having intensity variable in accordance with the intensity Of the received signal and the other of said de.- vices having a control electrode, a source of operating voltage, connections for said anode current and'said control electrode to said source of operating voltage, a resistance in the connection for said anode current, a resistance in said control electrode connection, and means to maintain a substantially constant potential between the termini of said two resistances remote from said source of operating potential during the variations in intensity of received signals encountered during practice and during absence of received signals, and means including said last means to produce a bias on said control electrode satisfactory for operation of said other discharge device when said anode current is of a val'ue corresponding to the intensity of signals to be received and such as to render said discharge device substantially inoperative when said anode current has a value corresponding to less than a predetermined value of received signals.

13. The combination, in a radio receiver, of an amplier for received signals having a control electrode, a circuit having a point thereon at which the potential with respect to lground varies during variation in intensity of received signals over a range suicient in width forcontrol oi said amplifier if applied to said control electrode but at potentials with respect to ground diiierent from desired potentials of said control electrode, a constant voltage gaseous discharge device having one electrode connected to said point and another electrode connected to said control electrode, means to maintain conduction between said electrodes both during absence and presence of received signals, said gaseous discharge device being adapted to maintain substantially constant the voltage between said electrodes during all values of potential on said point with respect to ground, whereby substantially the entire range of variation in voltage of said point is supplied to potential of said control electrode has a value at one end of said range and inoperative when said potential has a value at the other end of said range.

14. In combination, .an amplier having a pair of input electrodes connected through respective resistances to ground, means to maintain said electrodes at certain low potentials with respect to ground, a discharge device having an output circuit having a point thereon at which the potential varies through a range relatively high with respect to ground and of sufficient width for control of said amplifier if applied to one of said control electrodes, means connected between said point and said one input electrode to maintain a constant potential therebetween during said variations in potential, and means connected between said input electrodes operative when said range is exceeded to produce a voltage on the other input electrode to oppose the voltage on said one input electrode.

15. The combination, in a signal receiver, of a circuit having a point thereon at which the potential with respect to ground varies with variations in received signal intensity, an amplifier having a cathode and a control electrode, a circuit between said control electrode and cathode including a connection between said control electrode and said point, means to vary the voltage between said cathode and said control electrode over such a range that said amplifier is inoperative in the absence of received signals and operativeduring received signals, and means responsive to change in polarity of electromotive force between said point and a second point having potential not to be exceeded by the potential at said iirst point to limit the variation in voltage between said grid and cathode upon reception of strong signals.

16. In combination, an amplifier having an anode circuit including a resistance and a control electrode circuit including said resistance, both of said circuits including a cathode of said ampliiier, means to supply to the control electrode of said amplier electromotive forces to be amplified thereby and a bias potential, said bias potential varying through a range of values extending in the direction of the potential of said cathode from a value so negative relative to the potential of said cathode that the amplification of said amplier is substantially reduced thereby, and means responsive to changes in the diierence of potential between said control electrode and a point on said resistance to limit the bias voltage applied to said control electrode.

17. In combination, an amplier having an anode circuit including a resistance and a control electrode circuit including said resistance, both of said circuits including a cathode of said amplifier, means to supply to the control electrode of said amplifier electromotive forces to be amplied thereby and a bias potential, said bias potential varying through a range of values extending in the direction of the potential of said cathode from a value so negative relative to the potential of said cathode that the ampliiication of said ampliiier is substantially reduced thereby, and a unilateral conducting device connected between said control electrode and a point on said resistance, said device being so poled and said point being so selected that said device passes current, only when the potential of said control electrode differs from said value by more than a predetermined amount.

GEORGE M. BROWN.

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